[lldb] Remove ClangASTMetrics

Summary: Not once have I looked at these numbers in a log and considered them useful. Also this should not have been implemented via an unguarded list of globals.

Reviewers: martong, shafik

Reviewed By: shafik

Subscribers: rnkovacs, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71336